So called oscillatory forks (e.g. EP 0 444 173 B1), single rods (e.g. WO 2004/094964 A1) or also membrane oscillators are known from the state of the art for determining fill level and other process variables of a medium. The characterizing variables of the mechanical oscillations (oscillation amplitude, resonance frequency, phase response as a function of frequency) of the oscillatable unit depend on contact with the medium and also its properties. This is exploited for the various measurements. Thus for example, the frequency or the amplitude of the oscillations decreases especially when a liquid medium reaches and at least partially covers the oscillatable unit. On the one hand, the liquid medium acts on the oscillating body of the sensor, that is. e.g. the oscillatory fork, the single rod, or the membrane, as a co-moving mass, so that the oscillation frequency sinks, and, on the other hand, as a mechanical damper, so that the oscillation amplitude decreases. Therefore, it can be ascertained from the decrease of the oscillation frequency or amplitude, dependent on the embodiment and the position of the application of the apparatus, that the medium has reached a fill level. Furthermore, the oscillation frequency is also dependent, for example, on the viscosity of the medium (see e.g. EP 1 325 301). Such measuring devices are most often excited to the resonance frequency oscillations by means of an electromechanical transducer.
Frequently, these sensors are used as limit level switches. If the process variable is, for example, the fill level, then the sensor produces a signal, which displays that the fill level, which is predetermined by the embodiment of the sensor and its location of mounting, was reached or subceeded (fallen beneath).
In the use of oscillatory forks in liquids with low density (e.g. smaller than 0.5 g/cm3 such as liquefied gases), the frequency difference between the oscillation frequency in air and the oscillation frequency in the immersed state is usually too small for an assured evaluation. The temperature and the process pressure influence especially the oscillation frequency in similar orders of magnitude as a frequency change as a result of a change of the density of the medium. In other words: the density sensitivity of the sensor can be too small in such cases in comparison to disturbing influences such as temperature or density.